Voltage clamp experiments are carried out to determine the function and structure of ionic channels in single giant nerve and muscle fibers. The project seeks to obtain information about the position and properties of membrane ionic channel sites that limit conductances and determine ionic selectivity by an analysis of the interaction between current-carrying and blocking ions. The ability of various kinetic models to describe the flow of ions through open channels is tested. Alkali metal and transition metal lanthanide ions are employed in this study. Tests are run to determine whether "screening " or "binding" hypotheses describe the lanthanide blocking effects. Analogies between the kinetics of ionic currents across the nerve membrane and those seen in excitable lipid bilayer membrane preparations treated with channel forming agents are investigated. Divalent ion currents and fluxes in nerve membrane channels are investigated. A determination of the properties of the gating current in ionic channels in single muscle fibers is an integral part of this project. The voltage dependence of the gating current is sought.